Fingerprint sensor in InFO structure and formation method

ABSTRACT

A package includes a sensor die, and an encapsulating material encapsulating the sensor die therein. A top surface of the encapsulating material is substantially coplanar with or higher than a top surface of the sensor die. A plurality of sensing electrodes is higher than the sensor die and the encapsulating material. The plurality of sensing electrodes is arranged as a plurality of rows and columns, and the plurality of sensing electrodes is electrically coupled to the sensor die. A dielectric layer covers the plurality of sensing electrodes.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the following provisionally filedU.S. Patent application: Application Ser. No. 62/349,355, filed Jun. 13,2016, and entitled “Fingerprint sensor in InFO structure and FormationMethod;” which application is hereby incorporated herein by reference.

BACKGROUND

Fingerprint sensors are used to capture the fingerprint images. Thefingerprint images may be used for identification purpose. For example,the captured fingerprint images may be searched in a database in orderto match with pre-stored fingerprint images.

There are several ways of capturing fingerprint images, includingoptical, ultrasonic, passive capacitance, active capacitance, etc. Inthe passive capacitance and active capacitance methods, capacitancesensors use principles associated with capacitance in order to formfingerprint images. In these methods of imaging, sensor arrays are used.Each of the sensing electrodes in a sensor array acts as one plate of aparallel-plate capacitor. The dermal layer (which is electricallyconductive) of the finger to be sensed acts as the other plate, and thenon-conductive epidermal layer of the finger acts as a dielectric layer.The differences between the capacitance values of the sensing electrodesin the array reveal the ridges and valleys in the fingerprint.

A conventional fingerprint sensor includes a sensor die having aplurality of surface bond pads at a top surface. The bond pads arebonded to printed circuit boards etc. through wire bonding. Thefingerprint sensor may further include a sensor array at the topsurface, wherein the sensor array includes a plurality of sensingelectrodes. The sensor die is molded in a molding compound, which coversand protects the bond wires and the sensor array.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 11B are cross-sectional views of intermediate stages inthe formation of sensor packages in accordance with some embodiments.

FIG. 12 is a top-view of an exemplary sensor package in accordance withsome embodiments.

FIG. 13 is a cross-sectional view of a sensing unit in accordance withsome embodiments.

FIGS. 14 through 21 illustrate the cross-sectional views of intermediatestages in the formation of fingerprint sensor packages in accordancewith some embodiments.

FIG. 22 illustrates a process flow for forming a sensor package inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A sensor package and the method for forming the sensor package areprovided in accordance with various exemplary embodiments. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements. It is appreciated that although fingerprintsensor packages are used as example, other types of sensor packagesother than capturing fingerprint images may also be formed.

FIGS. 1 through 11B illustrate the cross-sectional views of intermediatestages in the manufacturing of a sensor package in accordance withembodiments. The steps shown in FIG. 1 through 11B are also illustratedschematically in the process flow 200 shown in FIG. 22.

FIG. 1 illustrates carrier 20 and release layer 22 formed on carrier 20.Carrier 20 may be a glass carrier, a ceramic carrier, or the like.Carrier 20 may have a round top-view shape, and may have a size of asilicon wafer. For example, carrier 20 may have an 8-inch diameter, a12-inch diameter, or the like. Release layer 22 may be formed of apolymer-based material (such as a Light To Heat Conversion (LTHC)material), which may be removed along with carrier 20 from the overlyingstructures that will be formed in subsequent steps. In accordance withsome embodiments of the present disclosure, release layer 22 is formedof an epoxy-based thermal-release material. Release layer 22 may becoated onto carrier 20. The top surface of release layer 22 is leveledand has a high degree of co-planarity.

Dielectric layer 24 is formed on release layer 22. In accordance withsome embodiments of the present disclosure, dielectric layer 24 isformed of a polymer, which may also be a photo-sensitive material suchas polybenzoxazole (PBO), polyimide, benzocyclobutene (BCB), or thelike, that may be easily patterned using a photo lithography process. Inaccordance with alternative embodiments, dielectric layer 24 is formedof a nitride such as silicon nitride, an oxide such as silicon oxide,PhosphoSilicate Glass (PSG), BoroSilicate Glass (BSG), Boron-dopedPhosphoSilicate Glass (BPSG), or the like.

Redistribution Lines (RDLs) 26 are formed over dielectric layer 24. Therespective step is shown as step 202 in the process flow shown in FIG.22. RDLs 26 are also referred to as backside RDLs since they are locatedon the backside of sensor die 36 (FIG. 4). The formation of RDLs 26 mayinclude forming a seed layer (not shown) over dielectric layer 24,forming a patterned mask (not shown) such as a photo resist over theseed layer, and then performing a metal plating on the exposed seedlayer. The patterned mask and the portions of the seed layer covered bythe patterned mask are then removed, leaving RDLs 26 as in FIG. 1. Inaccordance with some embodiments of the present disclosure, the seedlayer includes a titanium layer and a copper layer over the titaniumlayer. The seed layer may be formed using, for example, Physical VaporDeposition (PVD). The plating may be performed using, for example,electro-less plating.

Referring to FIG. 2, dielectric layer 28 is formed on RDLs 26. Thebottom surface of dielectric layer 28 may be in contact with the topsurfaces of RDLs 26 and dielectric layer 24. In accordance with someembodiments of the present disclosure, dielectric layer 28 is formed ofa polymer, which may be a photo-sensitive material such as PBO,polyimide, BCB, or the like. In accordance with alternative embodiments,dielectric layer 28 is formed of a nitride such as silicon nitride, anoxide such as silicon oxide, PSG, BSG, BPSG, or the like. Dielectriclayer 28 is then patterned to form openings 30 therein. Hence, someportions of RDLs 26 are exposed through the openings 30 in dielectriclayer 28.

Referring to FIG. 3, metal posts 32 are formed. The respective step isshown as step 204 in the process flow shown in FIG. 22. Throughout thedescription, metal posts 32 are alternatively referred to asthrough-vias 32 since metal posts 32 penetrate through the subsequentlyformed molding material. In accordance with some embodiments of thepresent disclosure, through-vias 32 are formed by plating. Through-vias32 are used for electrically inter-coupling the conductive features onthe opposite ends of through-vias 32. The formation of through-vias 32may include forming a blanket seed layer (not shown) over layer 28 andextending into openings 30 (FIG. 2), forming and patterning photo resist33, and plating through-vias 32 on the portions of the seed layer thatare exposed through the openings in photo resist 33. Photo resist 33 isthen removed, and some portions of the seed layer are exposed. Theexposed portions of the seed layer are then removed. The material ofthrough-vias 32 may include copper, aluminum, or the like. The top-viewshapes of through-vias 32 may be circles, rectangles, squares, hexagons,or the like. In accordance with some embodiments of the presentdisclosure, through-vias 32 are arranged to align to a ring (in the topview of the structure in FIG. 3) encircling a region therein, whereinthe region is used for placing sensor die 36 (FIG. 4).

FIG. 4 illustrates the placement of sensor die 36. The respective stepis shown as step 206 in the process flow shown in FIG. 22. Sensor die 36is attached to dielectric layer 28 through Die-Attach Film (DAF) 38,which is an adhesive film. Sensor die 36 includes circuits that arerelated to the sensing of capacitance values and the processing of thesensed capacitance values. For example, sensor die 36 includes circuits59 (FIG. 13) that are used to generate fingerprint images and to enhancethe quality of the images, etc. Sensor die 36 may also includeencryption circuits, memories, and/or the like.

In accordance with some exemplary embodiments, metal pillars 42A and 42B(such as a copper pillars, collectively referred to as metal pillars 42)are pre-formed as the topmost portion of sensor die 36, wherein metalpillars 42 are electrically coupled to the integrated circuit devicessuch as transistors (not shown) in sensor die 36 through metal pads 40.In accordance with some embodiments of the present disclosure, a polymerfills the gaps between neighboring metal pillars 42 to form topdielectric layer 44. Top dielectric layer 44 may also include a portioncovering and protecting metal pillars 42. Polymer layer 44 may be formedof PBO in accordance with some embodiments of the present disclosure.

Sensor die 36 further includes an interconnect structure, which includesmetal pads/lines (not shown) and metal vias formed in Inter-MetalDielectric (IMD) layers (not shown). The IMD layers may be formed oflow-k dielectric materials, which may have dielectric constants (kvalues) lower than about 3.0, lower than about 2.5, or even lower.

Metal pillars 42 may include metal pillars 42A and metal pillars 42B.Metal pillars 42A may form an array including a plurality of rows andcolumns in a top view of the structure, or may be arranged with a layouthaving a repeating pattern such as a beehive pattern. Metal pillars 42Amay have a uniform size, a uniform top-view shape, a uniform top-viewarea, a uniform row pitch, and a uniform column pitch in accordance withsome exemplary embodiments of the present disclosure. Metal pillars 42Bmay be aligned to one or a plurality rings that encircles metal pillars42A.

Next, encapsulating material 48 is used to encapsulate sensor die 36 andmetal posts 32, as shown in FIG. 5. The respective step is shown as step208 in the process flow shown in FIG. 22. Encapsulating material 48fills the gaps between neighboring through-vias 32 and the gaps betweenthrough-vias 32 and sensor die 36. Encapsulating material 48 may includea molding compound, a molding underfill, an epoxy, and/or a resin. Thetop surface of encapsulating material 48 is higher than the top ends ofmetal pillars 42. Encapsulating material 48 may include a base material,which may be a polymer, a resin, an epoxy, or the like, and fillerparticles (not shown) in the base material. The filler particles may bedielectric particles of SiO₂, Al₂O₃, or the like, and may have sphericalshapes.

In a subsequent step, a planarization such as a Chemical MechanicalPolish (CMP) step or a mechanical grinding step is performed to thinencapsulating material 48, until through-vias 32 and metal pillars 42are exposed. The respective step is also shown as step 208 in theprocess flow shown in FIG. 22. Due to the planarization, the top ends ofthrough-vias 32 are substantially level (coplanar) with the top surfacesof metal pillars 42, and are substantially coplanar with the top surfaceof encapsulating material 48.

FIGS. 6 through 8 illustrate the formation of front-side RDLs, sensingelectrodes, and the respective dielectric layers. Referring to FIG. 6,dielectric layer 50 is formed. The respective step is shown as step 210in the process flow shown in FIG. 22. In accordance with someembodiments of the present disclosure, dielectric layer 50 is formed ofa polymer such as PBO, polyimide, or the like. In accordance withalternative embodiments, dielectric layer 50 is formed of siliconnitride, silicon oxide, or the like. Openings 52 are formed indielectric layer 50 to expose through-vias 32 and metal pillars 42. Theformation of openings 52 may be performed through a photo lithographyprocess.

Next, referring to FIG. 7, Redistribution Lines (RDLs) 54 are formed toconnect to metal pillars 42 and through-vias 32. The respective step isshown as step 212 in the process flow shown in FIG. 22. RDLs 54 may alsointerconnect metal pillars 42B and through-vias 32. RDLs 54 includemetal traces (metal lines) over dielectric layer 50 and vias 56extending into dielectric layer 50. Vias 56 are connected tothrough-vias 32 and metal pillars 42B. In accordance with someembodiments of the present disclosure, RDLs 54 are formed in a platingprocess, wherein each of RDLs 54 includes a seed layer (not shown) and aplated metallic material over the seed layer. The seed layer and theplated material may be formed of the same material or differentmaterials.

In addition, at the same time RDLs 54 are formed, sensing electrodes 58are also formed in the same process. Each of sensing electrodes 58 isconnected to one of metal pillars 42A through one of vias 56 indielectric layer 50. Sensing electrodes 58 act as capacitor platesduring the sensing (of fingerprint, for example). Accordingly, each ofsensing electrodes 58 terminates in the respective RDL layer, and is notconnected to any metal/conductive features in the RDL layer and anymetal/conductive features over the RDL layer.

FIG. 12 illustrates a top view of sensing electrodes 58 and RDLs 54 inaccordance with some exemplary embodiments of the present disclosure. Asshown in FIG. 12, sensing electrodes 58 may form an array.Alternatively, sensing electrodes 58 may be arranged with otherrepeating layouts, for example, having a beehive pattern. Each sensingelectrode 58 is connected to an underlying metal pillars 42A through oneof vias 56, as shown in FIG. 7. Accordingly, sensing electrodes 58 andmetal pillars 42A have a one-to-one correspondence.

Referring to FIG. 8, in accordance with various embodiments, dielectriclayer 60 is formed to cover sensing electrodes 58 and RDLs 54. Therespective step is shown as step 214 in the process flow shown in FIG.22. Dielectric layer 60 in these embodiments may comprise a polymer(s)such as PBO, polyimide, BCB, or the like. Alternatively, dielectriclayer 60 may include non-organic dielectric materials such as siliconoxide, silicon nitride, silicon carbide, silicon oxynitride, or thelike. In accordance with some embodiments of the present disclosure,after the formation of dielectric layer 60, no opening is formed indielectric layer 60. Also, no conductive feature is formed over sensingelectrodes 58 in order to electrically connect to sensing electrodes 58and RDLs 54. Throughout the description, the structure formed on carrier20 is referred to as package 100, which may be a composite waferincluding a plurality of sensor dies 36 arranged as an array.

FIGS. 9 through 11 illustrate the remaining process in the formation ofthe sensor package. First, package 100 is de-bonded from carrier 20(FIG. 8), for example, by projecting a UV light or a laser on releaselayer 22, so that release layer 22 decomposes in response to the heat ofthe UV light or the laser. The resulting structure is shown in FIG. 9.Package 100 may be placed on tape 62, which is fixed on frame 64 thatforms a ring. Dielectric layer 60 may be adhered to tape 62.

Before or after the de-bonding of carrier 20, some parts of dielectriclayer 24 are removed to expose metal pads, which are parts of RDLs 26.As a result, openings 66 are formed in dielectric layer 24. Theformation of openings 66 may be achieved through laser burning.Alternatively, when dielectric layer 24 is formed of a light-sensitivematerial such as PBO or polyimide, the formation of openings 66 may alsobe achieved through a photo lithography process.

FIG. 10 illustrates the formation of solder regions 68. The respectivestep is shown as step 216 in the process flow shown in FIG. 22. Forexample, a printing step may be performed to print a solder paste inopenings 66, followed by reflowing the solder paste to form solderregions 68. Alternatively, solder regions 68 are formed by droppingsolder balls in openings 66 and then performing a reflow.

In subsequent steps, as show in FIG. 11A, a device die such asHigh-Voltage (HV) device die 70 is bonded to solder regions 68, whichare electrically coupled to sensor die 36 through RDLs 54. Package 100is flipped upside down, and glass plate 72 is attached to package 100through adhesive 74. Package 100 may then be singulated in a die-sawprocess to form a plurality of identical packages, each including aportion of package 100 as illustrated. The respective step is shown asstep 218 in the process flow shown in FIG. 22. Next, the singulatedpackage may be connected to other package components 76, which may be aPrinted circuit Board (PCB), a flex PCB, or the like. The resultingpackage shown in FIG. 11A is referred to as sensor package 102.

Package 102 may be used for capture fingerprint images, for example,when a finger is in contact with area 80 of glass plate 72, theplurality of sensing electrodes 58 forms a plurality of capacitors withthe corresponding overlying parts of the finger. The capacitance valuesvary depending on whether a sensing electrode 58 forms a capacitor witha ridge or a valley of the skin of the finger, and the capacitancevalues may be used to generate a fingerprint image. Accordingly, besensing capacitance values related to sensing electrodes 58, thefingerprint image is captured.

FIG. 11B illustrates package 102 in accordance with some embodiments.The package 102 in FIG. 11B is similar to the package 102 shown in FIG.11A, except there is a plurality of RDL layers on the front side ofsensor die 36. Accordingly, sensing electrodes 58 are formed in the topRDL layer, and some RDLs 54 are formed between, and interconnecting,sensing electrodes 58 and metal pillars 42. An advantageous feature ofthe embodiments in FIG. 11B is that the area of the sensing electrodearray is no longer limited by the size of sensor die 36. Rather, thesize of sensing electrode array may be adjusted (enlarged or shrunk, forexample) to suit to the size of fingers or the sensed object. Forming anadditional RDL layer underlying sensing electrodes 58 may alsoadvantageously increase the distance between sensing electrodes 58 andthe circuits 59 (such as transistors, FIG. 13) in sensor die 36, andhence reduces the interference of the circuits to sensing electrodes 58.

In accordance some embodiments, some RDLs 54 may form seal ring 84, asshown in FIGS. 11A and 11B, The seal ring 84 is formed in the front-sideRDLs, and is adjacent to the edges of package 102. An exemplary sealring 84 is shown in FIG. 12, which shows the top view of package 102 inaccordance with some embodiments.

As shown in FIG. 12, sensing electrodes 58 form an array, which issurrounded by RDLs 54. The possible positions of the boundaries ofsensor die 36 are marked as 36A and 36B, which are correspondent to theembodiments in FIGS. 11A and 11B, respectively. Accordingly, in theembodiments as shown by 36A, the top-view area of sensing electrodearray 158 (formed of sensing electrodes 58) is greater than the top-viewarea of sensor die 36. In the embodiments as shown by 36B, the top-viewarea of sensing electrode array 158 is smaller than the top-view area ofsensor die 36.

Each of sensing electrodes 58 forms a sensing unit 86 in combinationwith the underlying circuits, wherein sensor die 36 includes a pluralityof identical sensing units 86. An exemplary sensing unit 86 is marked inFIG. 12. FIG. 13 schematically illustrates a cross-sectional view ofsome components in sensing unit 86, which includes one sensing electrode58. As shown in FIG. 13, sensing electrode 58 is electrically connectedto processing circuit 59 (represented using a transistor), which, amongother functions, has the function of detecting the capacitance value ofthe corresponding capacitor having sensing electrode 58 as one of thecapacitor plates.

It is realized that the circuit 59 for performing the function specificto the corresponding sensing unit 86 occupies a small portion of thechip area overlapped by the corresponding sensing electrode 58. There isa large area 90 unused by circuit 59. In conventional sensor dies, area90 cannot be used. The reason is that in conventional sensor dies,sensing electrodes are formed at the surface of the respective sensordie. The sensor die, however, is thin, and the vertical distance D1between the sensing electrodes and the circuit (active devices) issmall. As a result, if circuits are built in the unused area, thecircuits will have high interference to the overlying sensing electrode,and will affect the measured capacitance values. In the embodiments ofthe present disclosure, by forming sensing electrodes in RDLs that ishigher than the sensor die, distance D2 between sensing electrodes 58and the circuits is increased, sometimes to three times distance D1 orgreater. Accordingly, area 90 may be used to form circuits since theinterference of the circuits to sensing electrodes is reduced to anacceptable level due to the increased distance D2.

FIGS. 14 through 21 illustrate cross-sectional views of intermediatestages in the formation of a fingerprint sensor package in accordancewith some embodiments of the present disclosure. Unless specifiedotherwise, the materials and the formation methods of the components inthese embodiments are essentially the same as the like components, whichare denoted by like reference numerals in the embodiments shown in FIGS.1 through 11B. The details regarding the formation process and thematerials of the components shown in FIGS. 14 through 21 may thus befound in the discussion of the embodiments shown in FIGS. 1 through 11B.

Referring to FIG. 14, release layer 22 is coated on carrier 20, anddielectric layer 60 is formed over release layer 22. Next, RDLs 54 andsensing electrodes 58 are formed over dielectric layer 60. RDLs 54 andsensing electrodes 58 are formed in dielectric layer 50 in accordancewith some embodiments.

Next, referring to FIG. 15, sensor die 36 is bonded to sensingelectrodes 58 through solder regions 92. Sensing electrodes 58 are thusin physical contact with solder regions 92. Underfill 94 is then filledinto the gap between sensor die 36 and dielectric layer 50.

Referring to FIG. 16, through-vias 32 are formed through plating. Themethod of forming through-vias 32 is essentially the same as in theembodiments shown in FIGS. 3 and 4, and is not repeated herein. Next,sensor die 36 and through-vias 32 are encapsulated in encapsulatingmaterial 48, followed by a planarization step, as shown in FIG. 17. As aresult of the planarization, through-vias 32 are exposed. In accordancewith some embodiments of the present disclosure, semiconductor substrate96 of sensor die 36 is exposed after the planarization. In accordancewith alternative embodiments of the present disclosure, a layer ofencapsulating material 48 is left on top of semiconductor substrate 96,wherein the remaining layer of encapsulating material 48 is illustratedas 48′. Encapsulating layer 48′ is illustrated using a dashed line toindicate it may or may not exist in accordance with various embodiments.

Next, Referring to FIG. 18, dielectric layer 28 is formed overencapsulating material 48 and sensor die 36, followed by the formationof RDLs 26. In a subsequent step, as shown in FIG. 19, dielectric layer24 is formed to cover RDLs 26, and is then patterned to reveal the metalpads in RDLs 26. Solder regions 68 are formed to extend into theopenings in dielectric layer 24, and device die 70 is bonded to RDLs 26through solder regions 68, as shown in FIG. 20. Package 100 is thusformed.

In subsequent steps, package 100 is de-bonded from carrier 20, and glassplate 72 is adhered to package 100 through adhesive 74. Package 100 isthen singulated, and the resulting structure is shown as package 102 inFIG. 21. Package component 76, which may be a PCB or a Flex PCB, is thenconnected to solder regions 68.

The embodiments of the present disclosure have some advantageousfeatures. By forming sensing electrodes in RDLs rather than in sensordies, the sensing electrodes are spaced farther apart from the circuitsin the sensor die, and hence the circuits have lower interferences tothe sensing electrodes. In addition, the sensing electrodes in RDLs,being formed in the top RDL layer, are closer to the top surface of therespective package. Accordingly, the sensitivity of the sensing isimproved. In addition, the glass plate over the sensing electrodes mayhave an increased thickness without sacrificing the sensitivity. Thedurability of the sensor package is thus improved.

In accordance with some embodiments of the present disclosure, a packageincludes a sensor die, and an encapsulating material encapsulating thesensor die therein. A top surface of the encapsulating material issubstantially coplanar with or higher than a top surface of the sensordie. A plurality of sensing electrodes is higher than the sensor die andthe encapsulating material. The plurality of sensing electrodes isarranged as a plurality of rows and columns, and the plurality ofsensing electrodes is electrically coupled to the sensor die. Adielectric layer covers the plurality of sensing electrodes.

In accordance with some embodiments of the present disclosure, a packageincludes a sensor die including a plurality of metal pillars at a topsurface of the sensor die, and an encapsulating material encapsulatingthe sensor die therein. A top surface of the encapsulating material issubstantially coplanar with a top surface of the sensor die. A firstdielectric layer is over and contacting both the sensor die and theencapsulating material. A plurality of vias in the first dielectriclayer. A sensing electrode array is disposed over the encapsulatingmaterial. The sensing electrode array has a plurality of sensingelectrodes, each electrically coupled to a metal pillar in the pluralityof metal pillars through one of the plurality of vias. A seconddielectric layer has an upper portion over the sensing electrode array.

In accordance with some embodiments of the present disclosure, a methodincludes forming a metal post over a first dielectric layer, attaching asensor die to a top surface of the first dielectric layer, encapsulatingthe metal post and the sensor die in an encapsulating material,planarizing the encapsulating material to expose metal pillars of thesensor die and the metal post, forming a second dielectric layer overthe encapsulating material and the sensor die, and forming aredistribution layer. The redistribution layer includes a plurality ofsensing electrodes electrically coupling to the metal pillars, and aredistribution line electrically intercoupling the sensor die and themetal post. A third dielectric layer is formed over the redistributionlayer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a sensor die; anencapsulating material encapsulating the sensor die therein, wherein atop surface of the encapsulating material is substantially coplanar withor higher than a top surface of the sensor die; a plurality of sensingelectrodes over the sensor die and the encapsulating material, whereinthe plurality of sensing electrodes is arranged as a plurality of rowsand columns, and the plurality of sensing electrodes is electricallycoupled to the sensor die; a first dielectric layer covering theplurality of sensing electrodes; an underfill over and contacting thesensor die, wherein the underfill is encapsulated in the encapsulatingmaterial; and a plurality of solder regions in the underfill, whereinthe plurality of solder regions electrically connects the plurality ofsensing electrodes to the sensor die.
 2. The package of claim 1 furthercomprising: a second dielectric layer over and contacting the sensor dieand the encapsulating material, wherein the second dielectric layer isunderlying the plurality of sensing electrodes; and a plurality of viasin the second dielectric layer, wherein the plurality of viaselectrically connects the plurality of sensing electrodes to the sensordie.
 3. The package of claim 1 further comprising: a through-viapenetrating through the encapsulating material; and a redistributionline at a same level as the plurality of sensing electrodes, wherein theredistribution line is electrically coupled to the through-via.
 4. Thepackage of claim 1, wherein the sensor die comprises a plurality ofmetal pillars, and each of the plurality of sensing electrodes overlapsa corresponding metal pillar in the plurality of metal pillars.
 5. Thepackage of claim 1, wherein some of the plurality of sensing electrodesextend laterally beyond edges of the sensor die.
 6. The package of claim1 further comprising: an adhesive over and contacting the firstdielectric layer; and a glass plate attached to the adhesive.
 7. Thepackage of claim 1, wherein the encapsulating material comprises amolding compound.
 8. The package of claim 1 further comprising adie-attach film underlying and contacting the sensor die, wherein abottom surface of the die-attach film is coplanar with a bottom surfaceof the encapsulating material.
 9. The package of claim 1, wherein theencapsulating material comprises a portion encircling the sensor die,and side edges of the encapsulating material are in physical contactwith side edges of the sensor die.
 10. A package comprising: a sensordie comprising a plurality of metal pillars at a top surface of thesensor die; an encapsulating material encapsulating the sensor dietherein, wherein a top surface of the encapsulating material issubstantially coplanar with a top surface of the sensor die, and sideedges of the encapsulating material are in physical contact with sideedges of the sensor die; a through-via penetrating through theencapsulating material; a first dielectric layer over and contactingboth the sensor die and the encapsulating material; a plurality of viasin the first dielectric layer; a sensing electrode array over theencapsulating material, wherein the sensing electrode array comprises aplurality of sensing electrodes electrically coupled to the plurality ofmetal pillars through one of the plurality of vias; a redistributionline at a same level as the plurality of sensing electrodes, wherein theredistribution line electrically inter-couples the through-via and thesensor die; and a second dielectric layer having an upper portion overthe sensing electrode array.
 11. The package of claim 10, wherein thesecond dielectric layer further comprises a lower portion, and thesensing electrode array is in the lower portion of the second dielectriclayer.
 12. The package of claim 10, wherein the sensing electrode arrayextends laterally beyond edges of the sensor die.
 13. The package ofclaim 10, wherein the sensing electrode array has a top-view areasmaller than a top view area of the sensor die.
 14. The package of claim10, wherein the encapsulating material comprises a molding compound. 15.A method comprising: forming a metal post over a first dielectric layer;attaching a sensor die to a top surface of the first dielectric layer,wherein the sensor die is configured to capture fingerprint images;encapsulating the metal post and the sensor die in an encapsulatingmaterial; planarizing the encapsulating material to expose metal pillarsof the sensor die and the metal post; forming a second dielectric layerover the encapsulating material and the sensor die; forming aredistribution layer comprising: a plurality of sensing electrodeselectrically coupling to the metal pillars; and a redistribution lineelectrically intercoupling the sensor die and the metal post; andforming a third dielectric layer over the redistribution layer.
 16. Themethod of claim 15 further comprising forming a plurality of vias in thesecond dielectric layer, wherein the plurality of vias electricallycouples the plurality of sensing electrodes to the metal pillars. 17.The method of claim 16, wherein the plurality of sensing electrodes andthe vias are formed in a same plating process.
 18. The method of claim15 further comprising attaching a glass plate to a top surface of thethird dielectric layer through an adhesive.
 19. The method of claim 18,wherein the third dielectric layer is in physical contact with both theadhesive and the plurality of sensing electrodes.
 20. The method ofclaim 15, wherein after the sensor die is encapsulated in theencapsulating material, the encapsulating material comprises a portionencircling the sensor die, and side edges of the encapsulating materialare in physical contact with side edges of the sensor die.